1. Field of the Invention
The present invention relates to a method for setting a working origin for machining with a rotary tool relative to a work piece to be machined, and to a machine tool for implementing the same method.
2. Description of the Related Art
Generally, in machine processing of a work piece using a cutting tool, each time a cutting tool and the work piece is exchanged, it is required to reset the starting position of the contact between the cutting tool and the work piece to be machined. In particular, when a metal die is to be machined with a rotary tool such as an end mill, higher accuracy in machining (dimensional accuracy, precision of finished surface) on the order of microns, or even on the order of nanometers, is required, and therefore, it is necessary to set the working origin with higher accuracy.
Setting a tool and a work piece to be machined is difficult and has to be done with utmost care while the feed shaft is being moved in a jog feed, which result in a very time consuming operation, and therefore requires a lot of experience and skill. Standardization of this operation is very difficult and there is a limitation in increasing work efficiency.
A method for automating this operation and setting the working origin efficiently in a short time is disclosed in Japanese Patent Publication No. 2004-98213. This method requires a special measuring apparatus including a sensor, which increases costs. In addition, the sensing surface is provided in a direction perpendicular to a rotation axis of the tool, although a position in the direction of the rotation axis of the tool can be detected, it is not possible to detect a position in the direction perpendicular to the rotation axis, in other words, in the radial direction of the tool. Further, this method can only be applied to a small diameter tool with an outer diameter not greater than 100 μm, and therefore applicable range of the method is limited.
As another method in prior art, a method is known in which a dedicated device provided with a microscope for detecting a tool is used. With this tool detection device, there is a problem that, although the position of a tool can be detected, the position of the work piece to be machined cannot be detected. Further, there is a problem that, when a tool with a sharp cutting edge is used, the position of the tip of the cutting edge cannot be accurately detected. In other words, when a microscope is used, the distance between the cutting edge and the work piece can be measured so as to bring the tool as close as possible to the work piece, but depending on the machining arrangement, it may be impossible to bring the cutting edge into view due to the focal length of the microscope.